Over the years, various arrangements have been developed to effect movement of different parts of a mobile hospital bed with respect to each other. For example, upward and downward movement of a patient support litter relative to a base has been effected with a pair of spaced hydraulic cylinders which have the cylinder housings fixedly mounted on the base and which have vertically extending piston rods with their upper ends fixedly secured to the patient support portion. However, hydraulic arrangements tend to drip oil, which creates a mess and which in some cases presents a safety problem when the oil ends up on a floor surface where someone may slip on it. In order to be competitive in today's marketplace, a hydraulic arrangement must usually include both electrically and manually actuated pumps, which tends to render the overall hydraulic system rather complex and expensive. Further, if the cylinders each have only a single piston, then in order for the lowest position of the patient support litter to be reasonably low, the lower end of the hydraulic cylinder must be mounted relatively close to the floor, which increases the chance it may fail to clear an obstruction when the mobile bed is being moved, resulting in possible damage to the hydraulic cylinder and/or a need to manually lift the bed over the obstruction.
As a known alternative to hydraulic cylinders, the patient support litter is sometimes supported on the base by a scissors mechanism, which may be driven by a electric drive mechanism or by a single small hydraulic cylinder. In either case, the scissors mechanism has a number of potential points at which a finger or other body part could be caught and pinched, which can present a safety problem. Also, scissors mechanisms tend to be relatively complex and therefore expensive. Further, the vertical space required by a scissors mechanism between a patient support litter and a base tends to be sufficiently large that it is difficult to achieve a design in which the patient support litter can move to a relatively low position.
Beds often have other movable parts, such as a movable knee support section of a patient support assembly. Arrangements of this type are usually driven by an electric motor, and the most common approach is to fixedly support on the bed frame an electric motor having an elongate rotatable shaft which is threaded, to support a nut on the shaft for movement therealong relative to the frame, and to use a link mechanism to operationally couple the nut to the part to be moved.
An object of the present invention is to provide an electrically driven arrangement for effecting relative movement of two parts of a hospital bed which is relatively simple in structure and which is cheaper than known arrangements, and in particular which is suitable for effecting vertical movement of a patient support litter relative to a base.
A further object is to provide such an arrangement which has no serious pinch points and is thus safer than known scissors mechanisms.
A further object is to provide such an arrangement which, when used to movably support a patient support litter on a base, has a minimal vertical height in its collapsed position so that the patient support litter can be moved to a relatively low position with respect to the base, and which has a relatively large range of movement in comparison to known devices.
A further object is to provide such an arrangement which is durable and reliable.